lkml.org 
[lkml]   [2010]   [May]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: schedule() && prev/current (Was: [PATCH 3/3] Make get_current() __attribute__((const)))
    On 05/19, Peter Zijlstra wrote:
    >
    > On Tue, 2010-05-18 at 23:22 +0200, Oleg Nesterov wrote:
    >
    > > And, looking at this patch I think that schedule() can be simplified
    > > a little bit.
    > >
    > > "sched: Reassign prev and switch_count when reacquire_kernel_lock() fail"
    > > commit 6d558c3ac9b6508d26fd5cadccce51fc9d726b1c says:
    > >
    > > Assume A->B schedule is processing,
    > > ...
    > > Then on B's context,
    > > ...
    > > prev and switch_count are related to A
    > >
    > > How so? switch_count - yes, we should change it. But prev must be
    > > equal to B, and it must be equal to current. When we return from
    > > switch_to() registers/stack should be restored correctly, so we
    > > can do
    >
    > What if schedule() got called at a different stack depth than we are
    > now?
    >
    > I don't think we can assume anything about the stack context we just
    > switched to.

    Not sure I understand...

    OK. Firstly, we shouldn't worry about the freshly forked tasks, they
    never "return" from switch_to() but call ret_from_fork()->schedule_tail(),
    right?

    Now suppose that A calls schedule() and we switch to B. When switch_to()
    returns on B's context, this context (register/stack) matches the previous
    context which was used by B when it in turn called schedule(), correct?

    IOW. B calls schedule, prev == B. schedule() picks another task, prev
    is saved on B's stck after switch_to(). A calls schedule(), prev == A
    before context_switch(A, B), but after that switch_to() switches to
    B's stack and prev == B.

    No?


    I am looking into the git history now... and I guess I understand why
    reacquire_kernel_lock() uses current. Because schedule() did something
    like

    prev = context_switch(prev, next); // prev == last

    finish_task_switch(prev);

    reacquire_kernel_lock(current); // prev != current

    Oleg.



    \
     
     \ /
      Last update: 2010-05-19 12:33    [W:0.045 / U:92.140 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site